This invention relates in general to safety devices for firearms and deals more particularly with an improved safety mechanism for a magazine loaded firearm which disables the firearm when its magazine is removed. An automatic or semi-automatic firearm of the aforedescribed type is potentially dangerous after its magazine has been removed, since a cartridge may remain in its firing chamber. Heretofore various magazine operated safety devices have been provided for disabling the firing mechanism of such a firearm. A typical safety mechanism of the aforedescribed general type is illustrated and described in U.S. Pat. No. 4,016,669, for example, and includes a magazine operated slide which either blocks or unblocks the trigger of a firearm depending on whether the magazine is in or out of its magazine well. The present invention is concerned with an improved magazine safety mechanism of the aforedescribed general type. It is the general aim of the present inventon to provide an improved, compact magazine safety mechanism which has relatively few parts, is easy to assemble and disassemble and has positive automatic mechanical action.